Television camera imagers, i.e., pickup tubes and solid-state devices such as charge coupled devices (CCD's), recurrently produce a charge pattern therein in response to a light image incident on a photosensitive surface thereof during what are commonly called optical integration intervals. Generally, the optical integration intervals coincide with the television field trace intervals. It is known to include a light-blocking shutter between the lens and imager of a television camera to limit the integration interval to a fraction of the television field trace interval. The reduced time that light is incident on the imager makes it possible for the image reproduced from each field of the image-representative signal to have very little motion induced blurring. Consequently, television cameras such as those including shutters are well suited for use at sporting events so that images with reduced blur can be provided during slow motion image replay.
One problem which may result when using a shuttered television camera, and which may result to a lesser extent in television cameras without a shutter, is a field-to-field brightness variation (flicker) in the video signal derived from the imager. The flicker results from brightness variations in areas lit up by artificial illumination, such as in a sports arena. Such brightness variations are usually caused by the peak-to-peak alternating current (AC) voltage swings of the power source for the illumination. For example, if the artificial illumination comprises flourescent lighting powered by a 60 Hz AC power source, the illumination will have brightness variations at a 120 Hz rate. Since the television camera field rate is not synchronized to the illumination power source, flicker may result in the video signal developed by the camera.
In addition, the television camera may include a shutter. If, for example, the shutter allows only one five-hundredth of a second exposure during each television field interval, since the operation of the shutter is not synchronized with the illumination power source, the shutter passes light to the imager which more closely follows the brightness variations of the illumination. Thus, the video signal provided by the camera will have pronounced brightness variations occurring at a rate corresponding to the difference in frequency (i.e., beat) between the television field rate and the illumination brightness variation rate. FIG. 1 of the drawing illustrates this effect, wherein waveform 10 corresponds to the illumination variations of an artificial light source, such as a flourescent light, about an average value A. Since the camera shutter is not synchronized with the illumination variation rate, the amount of light passed by the shutter opening during successive shutter opening intervals (T.sub.1, T.sub.2, T.sub.3, etc.) follows the changes in the illumination.
Even in television cameras which do not include a shutter and which operate at a field rate of 60 Hz (in accordance with the NTSC television standard), if the AC power source is 60 Hz, brightness variations may occur at a relatively slow but nevertheless annoying rate, due to the aforementioned lack of synchronization. Furthermore, if a 60 Hz field-rate television camera without a shutter is used in an area having a 50 Hz power source for its illumination (such as in many European countries), a much faster and therefore even more annoying flicker will result.
Consequently, three-phase AC power is often used for artificial illumination in areas where television cameras may be used, and especially when such television cameras are of the shutter type. Three-phase power for lighting results in a more uniform illumination and, therefore, greatly reduces the potential for video signal flicker.
However, with the advent of small, lightweight, and therefore highly portable television cameras, and in particular cameras using solid-state imagers, television cameras are being used in a variety of uncontrolled illumination situations. Consequently, it is desirable to provide apparatus to reduce the possibility of field-rate flicker in the video signal provided by television cameras in such situations. Although brightness variations could be removed by a video AGC (automatic gain control) circuit in the camera, most commercial (broadcast quality) television cameras do not include AGC since such circuits can be easily fooled by small area illumination highlights, and prevent accurate reproduction of the scene illumination. Consumer type television cameras may include an AGC circuit and even an automatic iris, i.e., a lens iris which opens and closes in response to scene illumination. However, these features do not adequately solve the flicker problem because the time constant of the AGC and auto-iris circuits are typically on the order of one second (to avoid annoying rapid fluctuation of image brightness), while video signal flicker due to brightness variations induced by an artificial light source may be on the order of one-tenth of a second (e.g., when using an NTSC television camera in a European country) and therefore cannot be adequately controlled by the AGC or auto-iris circuits in consumer type cameras.